Sealable films

ABSTRACT

A film having superior seal strength and barrier properties comprises a base layer comprising 70 to 97 weight % of a polyolefin, e.g. polypropylene, and 3 to 30% weight of a resin of lower molecular weight than the polyolefin, e.g. a hydrogenated petroleum resin. On at least one of the surfaces of the base layer is a 1 to 20 weight % of a film of a random copolymer of 80 to 99 weight % of propylene and 1 to 20 weight % of ethylene, for example, an isotactic propylene-ethylene copolymer.

This invention relates to films based on polypropylene including thosewhich can be sealed by the application of heat.

Various polypropylene films have been described, e.g. in GB No.2028168A, EP No. 135178 and DE No. 3247988 where the core or substrateof the film is polypropylene. Various additives are added for examplecertain modulus improvers, certain antistatic media, siloxanes andpigments.

However, none of these documents teach that films may be obtained havinga seal strength which is significantly improved compared with previouslyknown polyolefin films. We have devised films having a seal layerneeding only one component which show improved seal strength propertiesover those previously known having only one component in the seal layer,i.e. either the seal strength is much improved at the same temperatureof sealing or the same seal strength can be obtained at lowertemperatures.

According to this invention a film, particularly a sealable filmcomprises a base layer comprising 70 to 97 wt. %, preferably 70 to 90weight % of a polyolefin and 3 to 30, preferably 10 to 30 weight % of aresin of lower molecular weight than that of the polyolefin, said layerhaving on at least one surface thereo 1 to 20 weight % based on theweight of the base layer, of a film layer comprising a random copolymerof 80 to 99 weight % of propylene and 1 to 20 weight % of ethylene.

These films are capable of being used in the packaging industry and canmeet the stringent criteria which are required for such films, such assufficiently high modulus and excellent clarity. In addition, they havegood barrier properties by which we mean low permeability to watervapour and gases, particularly oxygen. The films also have superior sealstrength properties and are especially useful in heat seal packagingalthough they are useful in other packing outlets where cellophane istraditionally used, such as twist wrapping.

The base layer comprises a polyolefin and a low molecular weight resin.The polyolefin is preferably a polymer of a mono-alpha olefin containingfrom 2 to 8 carbon atoms and particularly 2 to 4 carbon atoms permolecule. Thus the polymer may be a homopolymer of one of the followingolefins: ethylene, propylene, butene-1 and 4-methyl pentene-1, or hexeneor a copolymer of two or more of these olefins. A particularly suitablematerial for the base layer of the film is polypropylene, especially ahigh molecular weight stereoregular predominantly crystalline polymer ofpropylene. Alternatively, a copolymer of propylene with up to 20 weightpercent of another olefin, e.g. ethylene, could be used. An especiallypreferred polyolefin is an isotactic polypropylene with a density offrom 0.86 to 0.92 g/cc measured at 23° C. according to ASTM D 1505 and amelt flow index of from 1 to 15 g/10 min as determined according to ASTMD1238 (conditions 230° C. and 2.16 kg). It maybe made by Zieglerpolymerisation methods using for example AlCl₃ and TiCl₄ as catalyst.

The other component of the base layer is the low molecular weight resin,preferably a hydrogenated resin. This resin has a molecular weight belowthat of the polyolefin and is usually less than 5000, preferably lessthan 1000, for example, 500 to 1000. The resin can be a natural orsynthetic resin and can have a softening point as measured by ASTM E 28of from 60° to 180° C., e.g. from 80° to 150° C., most preferably from100° to 140° C., especially 120° C. to 140° C.

Suitable resins which can subsequently be hydrogenated are hydrocarbonresins, ketone resins, polyamide resins, colophonium, coumarone resins,terpene resins, chlorinated aliphatic or aromatic hydrocarbon resins.Examples of hydrocarbon resins are polymers of coke oven gas, crackednaphtha, gas oil and terpene oil.

Particularly preferred hydrogenated resins are hydrogenated petroleumresins. These are usually prepared by catalytically hydrogenating athermally polymerised steam cracked petroleum distillate fraction,especially a fraction having a boiling point of between 20° and 280° C.These fractions usually are of compounds having one or more unsaturatedcyclic rings in the molecule, such as cyclo dienes, cycloalkenes andindenes. It is also possible to hydrogenate resins produced by thecatalytic polymerisation of unsaturated hydrocarbons.

Before hydrogenation occurs the polymerised resin is usually dissolvedin a saturated hydrocarbon solvent such as heptane. The hydrogenationcatalysts used may be nickel, reduced nickel, or molybdenum sulphide.Hydrogenation can take place in a single stage at a temperature of 200°C. to 330° C., preferably from 210° C. to 230° C. at a pressure of 20 to120 atmospheres, more preferably from 30 to 90 atmospheres for a periodof 5 to 7 hours. After filtering off the catalyst, the solvent isremoved by distillation and recovered for recycling. An improvedhydrogenation process leading to increased yields of high qualityhydrogenated hydrocarbon resins is described and claimed in EuropeanPatent No. 0082726 (Application No.82306853.1).

The ratio of polyolefin to the lower molecular weight resin in the baselayer is 70 to 97, preferably 70 to 90 weight percent of polyolefin and3 to 30, preferably 10 to 30 weight percent of lower molecular weightresin. More preferably the relative amounts are respectively 75 to 85weight percent and 15 to 25 weight percent, for example about 80 weightpercent and 20 weight percent.

The base layer or core of the film has, on one or preferably bothsurfaces 1 to 20 weight percent, preferably 1 to 10 weight percent andparticularly about 5 weight percent based on the weight of the baselayer, a film layer of a copolymer of 80 to 99 weight percent ofpropylene and 1 to 20 weight percent of ethylene, such copolymers beingfrequently referred to as random copolymers. This copolymer comprisingthe film layer(s) preferably comprises 90 to 99 weight percent,especially 94 to 98 weight percent, e.g. about 95.5 weight percent ofpropylene, the remainder being ethylene and is especially useful toprovide heat-sealability.

This random copolymer of propylene and ethylene is usually an isotacticpropylene-ethylene copolymer with a density of from 0.86 to 0.92 g/ccmeasured at 23° C. according to ASTM D 1505 and a melt flow index offrom 2 to 15 g/10 min as determined according to ASTM D1238 (conditions230° C. and 2.16 kg). It may be made by well-known polymerisationmethods using a Ziegler catalyst, for example, AlCl₃ and TiCl₄.

The films of this invention, i.e. multilayer films are suitably formedby combining the components (which may if desired include othercomponents such as an antistatic medium, blocking agents and slip aids)of the base layer and surface layer(s) in a conventional manner,preferably by a simultaneous co-extrusion technique.

Multiple-layer films according to the invention may be unorientated oruniaxially orientated, but are preferably biaxially orientated bydrawing in two mutually perpendicular directions in the plane of thefilm to impart strength thereto. Orientation of flat film may beeffected by a stenter technique, while orientated tubular film issuitably produced by coextruding the polymeric materials in the form ofa multi-layer tube from an annular die, cooling the extruded tube (thecast tube), reheating and inflating the tube by the so-called "bubble"process to introduce transverse orientation, and simultaneouslyelongating the tube longitudinally to orientate the film in a lengthwisedirection. The film is then preferably "heat-set", i.e. dimensionalstability of the film is improved by heating the film, while restrainedagainst thermal shrinkage, to a temperature above the glass transitiontemperature of the polymer from which the film is formed but below themelting point thereof.

The films may vary in thickness depending on the intended application,but films of a thickness of from 2 to 150 microns are usually suitable.Films intended for packaging are usually from 10 to 60 microns thick.The thickness of each outer layer is usually from 0.05 to 2.5 microns.

EXAMPLE

In this Example coextruded films of different composition were made andcompared as to their sealability, stiffness, optical properties andbarrier properties.

These films were biaxially orientated by drawing in two materiallyperpendicular directions in the plane of the film to impart strengththereto. The stenter technique was used to impart an orientation ratioof 500% in machine direction and of 900% in transverse direction. Eachof the biaxially orientated films had two seal layers of 1 micronthickness while the total thickness was about 22 microns.

The first film which was for comparison purposes consisted of a core ofisotactic polypropylene having a density of about 0.90 g/cc measured at23° C. according to ASTM D 1505 and a melt flow index of 2.8 g/10 min asdetermined according to ASTM D1238 (conditions 230° C. and 2.16 kg). Onboth faces of the core there was a surface film of a random copolymer ofabout 95.5 weight percent propylene and 4.5 weight percent of ethylene.This was an isotactic copolymer with a density of 0.90 g/cc measured at23° C. according to ASTM D 1505 and a melt flow index of 6.5 g/10 min asdetermined according to ADTM D1238 (conditions 230° C. and 2.16 kg).

The second film was the same as the first film except that the core wasa blend of 80% by weight of the isotactic polypropylene used as the coreof the first film and 20% by weight of a hydrogenated petroleum resin.This resin was prepared by catalytically hydrogenating a thermallypolymerised steam cracked petroleum distillate fraction having a boilingpoint between 20° and 280° C. The hydrogenation process was carried outas described in European Patent No. 0082726 (Application No. No.82306853.1). The resulting resin was characterised by a Ring and Ballsoftening point of 125° C.

The accompanying FIGS. 1 to 7 show the results obtained for the sealstrength, hot tack, haze, gloss, stiffness and barrier properties forfilms 1 (comparative) and 2 (according to the invention)

These tests were carried out as follows:

Seal strength

This is the "cold" heat seal strength of the film which is measuredafter the seal has cooled to ambient temperature and the full potentialstrength of the seal has developed.

A Packforsk Hot Tack Tester (model 52-B) from Design & Test Consult AB(Bromma-Sweden) has been used to make the heat seals. A strip of 15 mmwide and some 280 mm long is thereto folded over upon itself and sealedtogether between the heated jaws of this instrument.

The formed seal has an area of some 15×5 mm. Dwell conditions on theseal have been kept constant at a pressure of 5 bar and a dwell time of0.5 seconds. To measure the seal strength, the instrument is used in amode whereby upon opening of the sealing jaws the automatic peelingaction used for hot tack measurements is not activated. The seal isinstead cooled to ambient temperature. The unsealed ends of the stripare then attached to the jaws of a tensile testing machine. The forcerequired to break the seal at a rate of 508 mm/min is recorded by thetensile tester and expressed as the seal strength in kg/15 mm.

Hot tack

This is the strength of the heat seal measured just after the seal hasbeen made and before the thermal energy employed to form the heat sealhas dissipated. The hot tack has been determined using the samePackforsk instrument. The heat seal is made as described above but justafter the opening of the sealing jaws a peeling action is nowautomatically started and the force to break the seal is automaticallyrecorded and expressed as the hot tack in kg/15 mm. The peelingoperation takes places at a controlled delay time (=time between openingof the jaws and start of the peeling operation) and controlled peelrate. Following constant conditions have been used for all hot tackdeterminations: delay time=0.9 seconds, peel rate=100 mm/s.

Haze and gloss

These optical properties of the film have been determined with a GardnerHazemeter, haze according to ASTM D1003 and gloss according to ASTMD523.

E-modulus

The modulus of elasticity which is a measurement of film stiffness hasbeen determined from the tensile test of 15 mm wide film samplesaccording to DIN 53457.

Barrier Properties

Humidity and oxygen barrier properties were determined by measuring thewater vapour transmission rate (WVTR) and the oxygen transmission rate(OTR). WVTR has been measured according to ASTM F 372 at 37.4° C. and100% relative humidity using a Permetran W5 instrument from ModernControls Incorporated. OTR was measured according to ASTM D 3985 at 27°C. using an Oxtran Twin instrument also from Modern ControlsIncorporated.

From FIG. 1 it can be seen that the seal strength (kg/15 mm) for film 2is much higher at temperatures of between 100° and 110° C. and above130° C. than for film 1. Between 110° and 130° C. the seal strength iscomparable or higher than for film 1.

From FIG. 2 it can be seen that the hot tack (kg/15 mm) for film 2 ismuch better than for film 1 for temperatures up to about 110° C.

From FIG. 3 and FIG. 4 it can be seen that there is less haze and moregloss for film 2 than with film 1.

From FIG. 5 it can be seen that the stiffness of film 2, as measured bythe E-modulus, is significantly higher than the stiffness of film 1.

From FIG. 6 and FIG. 7 it can be seen that the barrier properties offilm 2 are about twice as good as those of film 1.

It is quite surprising that with the presence of the relatively smallamount (20 weight %) of hydrogenated resin in the base layer oneachieves such better results compared with the prior art film.

We claim:
 1. A film comprising a base layer which comprises 70 to 97weight % of a polyolefin and 3 to 30 weight % of a hydrogenated resinhaving a molecular weight lower than that of the polyolefin, said layerhaving on at least one surface thereof 1 to 20 weight % based on theweight of the base layer, of a one component film layer of a copolymerof 80 to 99 weight % of propylene and 1 to 20 weight % of ethylene.
 2. Afilm according to claim 1 in which the base layer comprises 70 to 90weight % of the polyolefin and 10 to 30 wt. % of the resin.
 3. A filmaccording to claim 1 or claim 2 in which the copolymer is a randomcopolymer.
 4. A film according to claim 1 wherein the polyolefin in thebase layer is a polymer of a mono alpha olefin containing 2 to 4 carbonatoms per molecule.
 5. A film according to claim 4 wherein thepolypropylene is an isotactic polypropylene with a density of from 0.86to 0.92 g/cc measured at 23° C. according to ASTM D 1505 and a melt flowindex of from 1 to 15 g/10 min as determined according to ASTM D1238(conditions 230° C. and 2.16 kg).
 6. A film according to claim 5 whereinthe hydrogenated resin in the base layer is a hydrogenated petroleumresin.
 7. A film according to claim 6 wherein the hydrogenated resin hasbeen prepared by hydrogenating using a hydrogenation catalyst at atemperature of 200° to 330° C., a thermally polymerised steam crackeddistillate fraction or a catalytically polymerised unsaturatedhydrocarbon fraction.
 8. A film according to claim 7 in which thehydrogenated resin has a softening point as measured by ASTM E28 of 100°to 140° C.
 9. A film according to claim 1 wherein the base layercomprises 75 to 85 weight % of polyolefin and 15 to 25 weight % ofresin.
 10. A film according to claim 9 wherein the base layer has oneach surface 1 to 10 weight percent based on the weight of the baselayer of said film layer comprising said random copolymer.
 11. A filmaccording to claim 10 wherein the random copolymer comprises 94 to 98weight percent of propylene and 2 to 6 weight percent of ethylene.
 12. Afilm according to claim 11 wherein the random copolymer is an isotacticpropylene-ethylene copolymer with a density of from 0.86 to 0.92 g/ccmeasured at 23° C. according to ASTM D 1505 and a melt flow index offrom 2 to 15 g/10 min as determined according to ASTM D1238 (conditions230° C. and 2.16 kg).
 13. A film according to claim 12 which isbiaxially orientated.
 14. A film according to claim 13 which is 10 to 60microns thick and wherein the thickness of each outer film layer is 0.05to 2.5 microns.